Which MPI implementation for MPI-2? ...

Richard Walsh rbw at ahpcrc.org
Thu Apr 3 09:51:50 EST 2003


On Thu Apr  3 03:43:24 2003 Greg Lindahl wrote:

>On Wed, Apr 02, 2003 at 02:11:20PM -0600, Richard Walsh wrote:
>
>> Somewhat tangentially, but while we are on the subject of one-sided
>> communications in MPI-2, am I correct in assuming that this capability 
>> is implemented as it is in SHMEM ...
>
>No. It's much more complicated and general. You have to register
>windows within which one-sided ops can be used, and there are some
>extra calls that you make to make sure operations have completed.

 I see ... then I should also anticipate some loss of performance 
 (higher latency) when using one-sided MPI communications compared 
 to SHMEM. Or perhaps this is one-time overhead paid at registration
 only?

>UPC is a much more compact method of expressing one-sided calls, and
>unlike shmem, it can benefit from pipelined transfers.

 Right (so also with CAF) for messages, but you still have to explicitly 
 sychronize/lock, etc.

>> It would seem to be a requirement for speed and would 
>> also seem to require the use of identical binaries on each processor
>> (and COMMON or static to place data in a symmetric location).
>
>shmem doesn't require that; you can use a common address (I'm very
>punny at 1am) to exchange addresses of malloc-ed data. But with shmem,
>you get a free registration of all static & common variables, and the
>stack too, as long as you use it in a consistant fashion.

 As far as I know, SHMEM requires a known address either explicitly 
 passed (asymmetric location) between partners or a implicitly determined 
 from the symmetry relationships of the images communicating (static 
 or common).  As you say, this is "free" for COMMON/STATIC data.
 Perhaps we are actually agreeing ... explicitly exchange addresses
 of malloc-ed locations in different binaries would be fine.


 Thanks,

 rbw

 



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